function v. method

[danielx]

Duncan Booth wrote:
> danielx wrote:
>
> >>>> Foo.func = dan    # <-- Appearantly, something magical happens here,
> because...
> >>>> Foo.func
> ><unbound method Foo.dan>
> >>>> f = Foo.func
> >>>> f is dan               # <-- things begins to look suprising here.
> > False
> >>>> ismethod(f)
> > True
> >
> > Imagine my surprise. Why would Python do this?
> >
>
> Nothing magical happens at the point you said. The assignment is just an
> assignment and you saw that the function was stored unchanged when you
> looked in the class's dictionary.
>
> The magic happens when you access a member of a class or an instance. If
> the value is a descriptor then its __get__ method is called. Roughly
> speaking, Foo.func is equivalent to:
>
> >>> Foo.__dict__['func'].__get__(None, Foo)
> <unbound method Foo.dan>
>
> Python does this so that it can support other descriptor types such as
> property, classmethod, staticmethod.
>
> You can see this happening if you define your own descriptor:
>
> >>> class MyDesc(object):
>     def __get__(self, *args):
>         print "__get__ called", args
>         return None
>
>
> >>> d = MyDesc()
> >>> Foo.oops = d
> >>> Foo.oops
> __get__ called (None, <class __main__.Foo at 0x00B3F930>)
>
> http://docs.python.org/ref/descriptor-invocation.html has a description of
> this. Although it claims "Note that descriptors are only invoked for new
> style objects or classes (ones that subclass object() or type())." the
> descriptor mechanism is partially implemented for old style classes.
> Several aspects of descriptors don't work properly though in old-style
> classes which is one reason why you should always use new-style classes.
>
> > Privates don't have to be entirely absent from Klass.__dict__
> > (which would make Python not introspective); they can just be invisible
> > when using the dot-syntax.
>
> You could implement that using a data descriptor, but if you are going to
> prevent access to your private variables using the dot operator, then your
> code is going to look pretty silly with a lot of references to
> self.__dict__['theprivate'] which doesn't gain anything in readability over
> self.__theprivate.

I believe you are talking about the same thing as Leif, but I can't
quite tell. I'll need to read your paper as well :P.